1. Field of the Invention
The present invention relates to a semiconductor device and more particularly to a structure of a MOS (i.e., metal-oxide semiconductor) transistor.
2. Description of the Background Art
The gate electrode of a MOS transistor serves as part of wiring. In this respect, it is desirable to reduce the resistance of gate electrode lines as much as possible. However, the recent semiconductor devices continue to downsize to smaller dimensions. This forcibly narrows the width of respective gate electrode lines. The resistance of gate electrode lines increases correspondingly, inducing a problem that the undesirable voltage drop occurs or the response is delayed. To solve this problem, a conventionally proposed prospective method is to employ a poly-metal gate (i.e., a combination of polysilicon and metal) as the gate electrode.
FIG. 59 is a cross-sectional diagram showing the arrangement a MOS transistor having a poly-metal gate structure which is disclosed in Japanese Patent Application Laid-Open No. 2002-76336.
As shown in the drawing, source and drain regions 52 are selectively formed in an upper layer portion of a silicon substrate 51. Extensively overlying on the upper surface of the silicon substrate 51 is a SiO2 film 53 which serves as a gate oxide film. A doped polysilicon layer 54, formed on the SiO2 film 53, extends in a limited region corresponding to a gap between the source and drain regions 52. Formed on the doped polysilicon layer 54 is a tungsten layer 55. The doped polysilicon layer 54 and the tungsten layer 55 cooperatively constitute a poly-metal gate 50.
Smile Oxidation
An edge portion of the gate electrode partly overlaps with an impurity diffusion area of the source and drain regions 52 which has a relatively high impurity concentration. When a voltage of an accumulation direction is applied on the gate electrode, for example, when the gate voltage is smaller than the drain voltage in a NMOS transistor, there is a tendency that a leak current, so-called GIDL (i.e., gate induced drain leakage current), may flow in this overlap region.
FIG. 60 is a cross-sectional diagram explaining the GIDL phenomenon. As shown in this drawing, an inner depletion layer 61 and an outer depletion layer 62 appear along the boundary of the source or drain region 52 in accordance with the potential distribution. In this case, the upper edge of the inner depletion layer 61 develops under a region of the SiO2 film 53 located just beneath the doped polysilicon layer 54. The upper portion of depletion layer 61 elongated in this manner under the doped polysilicon layer 54 is a high electric field region 63 positioned adjacently to the edge of the doped polysilicon layer 54. The electric field of the gate edge portion can be simply expressed by {(Vgxe2x88x92Vd)/tOX}, where Vg represents a gate voltage, Vd represents a drain voltage, tOX represents a gate oxide film thickness. One of the methods for suppressing the GIDL phenomenon is a smile oxidation.
FIG. 61 is a cross-sectional diagram explaining the smile oxidation. The smile oxidation (also referred to as a poly-smile oxidation as a light thermal reoxidation) is a technique for forming a smile oxide film 56 by performing an oxidation process after forming a gate electrode. As shown in FIG. 61, the smile oxide film 56 has a large film thickness at the limited region near the gate edge.
Having a large film thickness at the region near the gate edge makes it possible for the smile oxide film 56 to relax the electric field in the vicinity of the gate edge portion. The GIDL phenomenon can be reduced correspondingly. Furthermore, the smile oxide film 56 has a relatively thin film thickness at the central region of the gate. Hence, the smile oxide film 56 can minimize the reduction in the drain current during an ON state.
Selective Oxidation
From the foregoing description, the ordinary person skilled in the art will simply expect that applying the smile oxidation process to the poly-metal gate structure may bring the effects of lowering the gate resistance and suppressing the GIDL phenomenon.
FIG. 62 is a cross-sectional diagram explaining a problem occurring when the smile oxidation process is applied to the poly-metal gate structure. Needless to say, it is well known that the metals, such as iron, copper, and aluminum, are readily oxidized as apparent from the generation of rust caused when the metals are oxidized.
In general, tungsten (W) is a metal material generally used for forming the poly-metal gate structure. However, compared with other metals, tungsten (W) is not an exception in that tungsten (W) easily bonds with oxygen to form an oxide having a higher resistance value. More specifically, as shown in FIG. 62, applying the smile oxidation process to the poly-metal gate structure causes the oxidation in the tungsten layer 55 and leaves an affected tungsten layer 55o. The resistance value of the poly-metal gate becomes large due to the presence of thus formed affected tungsten layer 55o. This is a problem to be solved in realizing an excellent poly-metal gate structure sufficiently low in the resistance value. From the view point that the capability of reducing the sheet resistance cannot be enjoyed when the metal is oxidized, this problem is fatal and will result in a negation of metal use.
To overcome this problem, there is known a conventional method according to which a selective oxidation technique is employed for performing the smile oxidation. The selective oxidation technique is characterized by an oxidation process performed in a strong reducing atmosphere, for example, containing a large amount of hydrogen. According to the selective oxidation technique, the oxide of tungsten (W), if it is once produced by the bonding of W and oxygen, promptly reduces to original W and oxygen. Thus, it becomes possible to minimize the chemical reaction to be caused between the tungsten (W) and oxygen.
However, the selective oxidation technique requires a sensitive or exquisite control for properly maintaining the balance between two directly-opposed, i.e., oxidation and reducing, phenomena. Accordingly, the required manufacturing or fabricating conditions are very severe. There is no degree of freedom. Simultaneously supplying both of oxygen and hydrogen significantly limits the temperature in the forming process for the purpose of avoiding possible dangers.
FIG. 63 is a cross-sectional diagram (Part I) explaining a problem peculiar to the selective oxidation process. As shown in FIG. 63, when the smile oxide film 56 having a sufficient thickness is formed in the vicinity of the edge, it is necessary to shorten the process time in view of the cost (or throughput) requirements. To this end, the process temperature needs to be maintained at a higher level. However, the above-described limit of the temperature in the forming process substantially prohibits the process being performed in such a higher temperature environment. On the other hand, if the supply of hydrogen is reduced to maintain the balance between the oxidation and reducing phenomena during the selective oxidation process, it will be unable to sufficiently suppress the oxidation of W.
From the foregoing reasons, as shown in FIG. 63, at least part of the tungsten layer 55 turns into the affected tungsten layer 55o as a result of oxidation. The gate resistance increases. In this manner, the selective oxidation technique requires a sensitive or exquisite control for properly maintaining the balance between two directly-opposed, i.e., oxidation and reducing, phenomena. From this fact, an upper limit of the film thickness of the smile oxide film, in the vicinity of the edge, is undesirably restricted to a smaller value. Furthermore, if the process time is elongated to obtain a sufficient film thickness under given conditions, it is needles to say that the throughput will be worsened.
FIG. 64 is a cross-sectional diagram (Part II) explaining a problem peculiar to the selective oxidation process. As shown in FIG. 64, the metal/poly interface (i.e., an interface between the doped polysilicon layer 54 and the tungsten layer 55) is also oxidized during the selective oxidation process. This induces a fatal problem that the interfacial resistance undesirably increases at the metal/poly interface.
It is an object of the present invention to provide a semiconductor device capable of realizing a low resistance gate electrode and also capable of reducing a leak current.
The present invention provides a semiconductor device including a semiconductor substrate, source and drain regions, a gate oxide film, a gate electrode, and an anti-oxidizing film. The source and drain regions are selectively formed in an upper layer portion of the semiconductor substrate. The gate oxide film is formed on the semiconductor substrate at least in a region between the source and drain regions. The gate electrode is formed on the gate oxide film and includes at least a polysilicon layer. The gate oxide film has a first region located beneath an edge of the gate electrode and a second region located beneath a center of the gate electrode. A film thickness of the first region of the gate oxide film is larger than a film thickness of the second region of the gate oxide film. The source and drain regions, the gate oxide film and the gate electrode cooperatively constitute a MOS transistor. The anti-oxidizing film has a small oxygen diffusion rate compared with the polysilicon layer, which covers the gate electrode so that the gate electrode is not exposed.
As the polysilicon layer is completely covered by the anti-oxidizing film, the first region of the gate oxide film located beneath the edge of the gate electrode becomes thick compared with the second region located beneath the center of the gate electrode. Thus, it becomes possible to suppress the oxidation in the polysilicon layer, even if the thermal treatment is performed to obtain the arrangement capable of suppressing the leak current.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.